Override a material on a Linked object (2.92)?
There are several ways to do this.
Using Object color.
This is quite limited way but it's easy.
You can use object color in node editor and the tricky thing is then you make an instance of the object it will use an object color of the instancer, replacing the original object colors.
So here is the workflow. You use an object color in the node tree:

Next, then you instance the object, use object color to set color for all objects inside the instance:

For this method, no proxy or overrides required, but you are limited to one RGB+Alpha color per instance.
Using Drivers
One of the solutions is using drivers. Let say, I have a character, and I'm going to drive the color of his clothes:

So I created a bone, that will drive this color. Then I pressed RMB on Y Location of this bone and select copy as new driver:

Next, I pasted this driver in Fac field:

Now, then I move this bone color makes blue.

That's all. Then you will use your character via proxy or library override, you will always make a local copy of pose, and changes in this pose will drive color in material.